To avoid the use of chlorine as a bleaching agent for pulp or other lignocellulosic materials, the use of ozone in the bleaching of chemical pulp has previously been attempted. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have previously limited the development of satisfactory ozone bleaching processes for lignocellulosic materials in general and especially for southern softwoods.
Ozone will readily react with lignin to effectively reduce the amount of lignin in the pulp, but it will also, under many conditions, aggressively attack the carbohydrate which comprises the cellulosic fibers of the wood to substantially reduce the strength of the resultant pulp. Ozone, likewise, is extremely sensitive to process conditions such as pH with respect to its oxidative and chemical stability. Changes in these process conditions can significantly alter the reactivity of ozone with respect to the lignocellulosic materials.
Since the delignifying capabilities of ozone were first recognized around the turn of the century, there has been substantial and continuous work by numerous persons in the field to develop a commercially suitable method using ozone in the bleaching of lignocellulosic materials. Furthermore, numerous articles and patents have been published in this area and there have been reports of attempts at conducting ozone bleaching on a non-commercial pilot scale basis. For example, U.S. Pat. No. 2,466,633 to Brabender et al., describes a bleaching process wherein ozone is passed through a pulp having a moisture content (adjusted to an oven dry consistency) of between 25 and 55 per cent and a pH adjusted to the range of 4 to 7.
Other non-chlorine bleach sequences are described by S. Rothenberg, D. Robinson & D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975)--Z, ZEZ, ZP and ZP.sub.a (P.sub.a -peroxyacetic acid); and N. Soteland, "Bleaching of Chemical Pulps with Oxygen and Ozone", Pulp and Paper Magazine of Canada, T153-58 (1974) --OZEP, OP and ZP. Further, U.S. Pat. No. 4,196,043 to Singh discloses a multi-stage bleaching process utilizing ozone and peroxide which also attempts to eliminate the use of chlorine compounds, and includes recycling of effluents.
Various bleaching apparatus utilizing a central shaft with arm members attached thereto are generally known (see, e.g., U.S. Pat. Nos. 1,591,070 to Wolf, 1,642,978 and 1,643,566, each to Thorne, 2,431,478 to Hill, and 4,298,426 to Torregrossa et al.). Also, U.S. Pat. Nos. 3,630,828 to Liebergott et al. and 3,725,193 to de Montigny et al. each disclose a bleaching apparatus for use with pulp having a consistency of above 15 percent, which apparatus includes a rotating shaft having radially spaced breaker arms for comminuting the pulp. Richter U.S. Pat. No. 4,093,506 discloses a method and apparatus for the continuous distribution and mixing of high consistency pulp with a treatment fluid such as chlorine or chlorine dioxide. The apparatus consists of a concentric housing having a cylindrical portion, a generally converging open conical portion extending outwardly from one end of the cylindrical portion, and a closed wall extending inwardly from the other end of the cylindrical portion. A rotor shaft mounted within the housing includes a hub to which a plurality of arms are attached. These arms are each connected to a transport blade or wing. Rotation of the shaft allows the treatment fluid to be distributed in and mixed with the pulp "as evenly as possible."
Fritzvold U.S. Pat. No. 4,278,496 discloses a vertical ozonizer for treating high consistency (i.e., 35-50%) pulp. Both oxygen/ozone gas and the pulp (at a pH of about 5) are conveyed into the top of the reactor to be distributed across the entire cross-section, such that the gas comes in intimate contact with the pulp particles. The pulp and gas mixture is distributed in layers on supporting means in a series of subjacent chambers. The supporting means includes apertures or slits having a shape such that the pulp forms mass bridges thereacross, while the gas passes throughout the entire reactor in intimate contact with the pulp.
Displacement of pulp through the reactor takes place by the repeated but controlled breaking of the supporting means by the rotation of the breaking means which are attached to and rotated by a central shaft. This allows the pulp to pass through the apertures and into the subjacent chambers. Fritzvold et al. U.S. Pat. No. 4,123,317 more specifically discloses the reactor described in the aforementioned Fritzvold '496 patent. This reactor also is used for treating pulp with an oxygen/ozone gas mixture.
U.S. Pat. Nos. 4,468,286 and 4,426,256 each to Johnson disclose a method and apparatus for continuous treatment of paper pulp with ozone. The pulp and ozone are passed along different paths either together or separately.
U.S. Pat. No. 4,363,697 illustrates certain screw flight conveyors which are modified by including paddles, cut and folded screw flights or combinations thereof for use in the bleaching of low consistency pulp with oxygen.
French Patent 1,441,787 and European patent Application 276,608 each disclose methods for bleaching pulp with ozone. European Patent Application No. 308,314 discloses a reactor for bleaching pulp with ozone utilizing a closed flight screw conveyor, wherein the ozone gas is pumped through a central shaft for distribution throughout the reactor. The pulp has a consistency of 20-50% and the ozone concentration of the treating gas is between 4 and 10% so that 2 to 8% application of ozone on O.D. fiber is achieved.
Despite all of the research conducted in this area, however, no commercially feasible process for the manufacture of ozone bleached lignocellulosic pulps from softwood and related pulps, especially southern softwood, has heretofore been disclosed, and numerous failures have been reported.
The present invention provides a novel apparatus and gaseous bleaching process which overcomes the problems encountered in the prior art as discussed herein to produce a high grade bleached pulp in a commercially feasible manner.